Video processing circuits providing enhanced detail in dark or light areas of displayed images are known. O. H. Shade, for example, describes such a processing circuit in U.S. Pat. No. 2,760,008 entitled AMPLIFIER HAVING CONTROLLABLE SIGNAL EXPANSION AND COMPRESSION CHARACTERISTICS which issued Aug. 30, 1956. The Shade processor includes so-called "black stretch" and "white stretch" non-linear processing circuits which provide enhanced visibility of picture detail in darker and lighter areas of displayed images. This is achieved by varying the amplification of the luminance signal as a function of the luminance signal level. For example, to improve detail in darker areas of a picture (hereafter, "black stretch" signal processing) the luminance signal is applied to a non-linear amplifier having a relatively higher gain for input signal levels in a range below about 50 IRE units. Similarly, improved detail brighter areas of a picture (hereafter, "white stretch" signal processing) increased gain is applied for input signals in a range above 50 IRE units.
Another example of non-linear luminance signal processing wherein a luminance signal is subjected to different degrees of amplification in different signal level ranges is described by T. Okada in U.S. Pat. No. 4,489,349 entitled VIDEO BRIGHTNESS CONTROL CIRCUIT which issued Dec. 18, 1984. The Okada apparatus includes a non-linear video processing circuit which provides variable gamma correction of luminance signal and in which the value of gamma is automatically determined by a control signal provided from an APL (average picture level) detector.